The invention relates generally to semiconductor devices, and deals more particularly with a semiconductor macro circuit which is electrically isolated from other adjacent semiconductor macro circuits integral therewith and other sources of external electrical noise.
There are many semiconductor technologies presently known. One common technology provides semiconductor devices produced in an epitaxially grown layer on a substrate of opposite conductivity or polarity. Another common technology provides semiconductor devices produced in a diffused layer.
External noise and internal feedback problems have arisen, particularly when the semiconductor devices form sensitive, high gain amplifiers. In such a case, a small amount of external noise or internal feedback can obscure the input signal or even drive the amplifier into oscillations. One route of the noise is laterally through the semiconductor epitaxially layer or diffused layer which contains the semiconductor devices or laterally through the lower substrate layer, to the input of a transistor or other sensitive device.
Heretofore, it was known to provide a single, low impedance isolation ring around a sensitive transistor i.e., a high gain (high beta) transistor used as an amplifier stage. For example, if the transistor is formed in an epitaxial layer, an isolation ring of opposite polarity to the epitaxial layer is diffused from the surface of the epitaxial layer to the substrate, and either floated or tied to a voltage. The voltage is ground voltage if the isolation ring is P type and positive power supply voltage if the isolation ring is N type. For a P type isolation ring, noise in the form of hole carriers traveling laterally through the epitaxial layer is shunted through the isolation ring to ground. For an N-type isolation ring, noise in the form of electron carriers travelling through the epitaxial layer is shunted through the isolation ring to the power supply voltage. Thus, the single isolation ring has proven effective in isolating the transistor from moderate levels of noise. If many sensitive transistors are provided in the same epitaxial or diffused layer, then each can be protected by its own single isolation ring.
Nevertheless, some noise hole carriers or electron carriers travel underneath the isolation ring through the substrate, and then interfere with the adjacent semiconductor devices. The noise problem does not dissipate until the noise hole carriers and electron carriers recombine, and this can slow the operation of the main device. Therefore, in high gain amplifiers, additional prior art precautions have been taken. The semiconductor chip containing an input stage/pre-amplifier can be physically separated by a spacing from the semiconductor chip containing the output/driver stage so that noise from the output/driver stage cannot feed back through the semiconductor layers to the input stage. While such a technique is effective in reducing the feedback, this technique is undesirable because it requires fabrication and interconnection of two separate chips.
Accordingly, a general object of the present invention is to provide a highly effective technique for isolating a stage of amplification or other macro device from external noise or internal feedback.
Another general object of the present invention is to provide a technique of the foregoing type which can be used to isolate different stages of a single amplifier yet permit the entire amplifier to be made from a single semiconductor chip.
A more specific object of the present invention is to provide an isolation technique which need not wait for recombination of hole and electron carriers.